[gjn]

I wouldn't get caught up in the car feeling slow until you take it to the track. The slow feeling may just be the fact that car isn't squating like it used to and the feeling during launch is different. I would be really suprised if the car was slower with the slide a links. Try to set the preload as best as possible. When I put the cal tracks on it didn't feel slower it just felt like the rear was lifting the whole car. I went to a tight 10" converter with 3,300 stall at the same time and it really woke up the combination. I also run a "highway" gear - 3.25s for the same reasons. As far as tire pressure, every car is different. I run 12 to 12.5 lbs in mine 26 x 10.5s. That is what it takes to get the best footprint. My car is really nose heavy, your car maybe different. Start out at about 16 lbs and drop in 1 lbs increments (if necessary) until you have even feathering across the tire.
I believe you will pick up with the DP too; however, I would get the combo "dialed in" from a slide a links standpoint before I made any other changes; otherwise, you won't know what cause what. I am running into this right now because I changed converters, carbs, and added cal tracks. I have to relearn my whole combo. I too run #16 heads that flow 250 at .5".

How hard are you spinning during launch at the track?